JP-3247: Retrieve pixel area from AREA reference file (#8187)

Co-authored-by: Howard Bushouse <[email protected]>

CHANGES.rst

@@ -62,6 +62,9 @@ photom
   the pipeline put the (variable, calculated per pixel) dispersion back in.  Assumes that
   the dispersion needs to be in Angstroms/pixel to match the required factor of ~10. [#8207]
 
+- Get the values of PIXAR_A2 and PIXAR_SR from AREA reference file
+  instead of PHOTOM reference file to avoid missmatching values. [#8187]
+
 - Added a hook to bypass the ``photom`` step when the ``extract_1d`` step 
   was bypassed and came before the ``photom`` step, e.g. for NIRISS SOSS
   data in the FULL subarray. [#8225]

docs/jwst/photom/main.rst

@@ -125,8 +125,9 @@ The step also populates the keywords PIXAR_SR and PIXAR_A2 in the
 science data product, which give the average pixel area in units of
 steradians and square arcseconds, respectively.
 For most instrument modes, the average pixel area values are copied from the
-primary header of the PHOTOM reference file.
-For NIRSpec, however,  the pixel area values are copied from a binary table
+primary header of the AREA reference file, when this file is available. Otherwise
+the pixel area values are copied from the primary header of the PHOTOM reference
+file. For NIRSpec, however, the pixel area values are copied from a binary table
 extension in the AREA reference file.
 
 NIRSpec IFU

jwst/photom/photom.py

@@ -255,6 +255,7 @@ def calc_nirspec(self, ftab, area_fname):
                 if row is None:
                     continue
                 self.photom_io(ftab.phot_table[row])
+
         # Bright object fixed-slit exposures use a SlitModel
         elif self.exptype == 'NRS_BRIGHTOBJ':
 
@@ -1150,6 +1151,30 @@ def create_1d_conversion(self, model, conversion, waves, relresps):
 
         return conversion, no_cal
 
+    def pixarea_from_ftab(self, ftab):
+        """
+        Short Summary
+        -------------
+        Read the pixel area values in the PIXAR_A2 and PIXAR_SR keys from the
+        primary header of the photom reference file.
+
+        Parameters
+        ----------
+        ftab : `~jwst.datamodels.JwstDataModel`
+            A photom reference file data model
+        """
+        area_ster, area_a2 = None, None
+        area_ster = ftab.meta.photometry.pixelarea_steradians
+        log.info('Attempting to obtain PIXAR_SR and PIXAR_A2 values from PHOTOM reference file.')
+        if area_ster is None:
+            log.warning('The PIXAR_SR keyword is missing from %s', ftab.meta.filename)
+        area_a2 = ftab.meta.photometry.pixelarea_arcsecsq
+        if area_a2 is None:
+            log.warning('The PIXAR_A2 keyword is missing from %s', ftab.meta.filename)
+        if area_ster is not None and area_a2 is not None:
+            log.info('Values for PIXAR_SR and PIXAR_A2 obtained from PHOTOM reference file.')
+        return area_ster, area_a2
+
     def save_area_info(self, ftab, area_fname):
         """
         Short Summary
@@ -1172,13 +1197,18 @@ def save_area_info(self, ftab, area_fname):
             Pixel area reference file name
         """
 
-        # Load the pixel area reference file
+        use_pixarea_rfile =  False
+        area_ster, area_a2 = None, None
         if area_fname is not None and area_fname != "N/A":
+            use_pixarea_rfile =  True
+            # Load the pixel area reference file
             pix_area = datamodels.open(area_fname)
 
-            # Copy the pixel area data array to the appropriate attribute
-            # of the science data model
-            if self.instrument != 'NIRSPEC':
+        if self.instrument != 'NIRSPEC':
+
+            if use_pixarea_rfile:
+                # Copy the pixel area data array to the appropriate attribute
+                # of the science data model
                 if isinstance(self.input, datamodels.MultiSlitModel):
                     # Note that this only copied to the first slit.
                     self.input.slits[0].area = pix_area.data
@@ -1190,21 +1220,29 @@ def save_area_info(self, ftab, area_fname):
                     self.input.area = pix_area.data[ystart: yend,
                                                     xstart: xend]
                 log.info('Pixel area map copied to output.')
-            else:
-                self.save_area_nirspec(pix_area)
-
-            pix_area.close()
 
-        # Load the average pixel area values from the photom reference file header
-        # Don't need to do this for NIRSpec, because pixel areas come from
-        # the AREA ref file, which have already been copied using save_area_nirspec
-        if self.instrument != 'NIRSPEC':
-            area_ster = ftab.meta.photometry.pixelarea_steradians
-            if area_ster is None:
-                log.warning('The PIXAR_SR keyword is missing from %s', ftab.meta.filename)
-            area_a2 = ftab.meta.photometry.pixelarea_arcsecsq
-            if area_a2 is None:
-                log.warning('The PIXAR_A2 keyword is missing from %s', ftab.meta.filename)
+                # Load the average pixel area values from the AREA reference file header
+                # Don't need to do this for NIRSpec, because pixel areas will be
+                # copied using save_area_nirspec
+                try:
+                    area_ster = pix_area.meta.photometry.pixelarea_steradians
+                    if area_ster is None:
+                        log.warning('The PIXAR_SR keyword is missing from %s', area_fname)
+                    area_a2 = pix_area.meta.photometry.pixelarea_arcsecsq
+                    if area_a2 is None:
+                        log.warning('The PIXAR_A2 keyword is missing from %s', area_fname)
+                    if area_ster is not None and area_a2 is not None:
+                        log.info('Values for PIXAR_SR and PIXAR_A2 obtained from AREA reference file.')
+
+                # The area reference file might be older, try the photom reference file
+                except AttributeError or KeyError:
+                    area_ster, area_a2 = self.pixarea_from_ftab(ftab)
+
+                pix_area.close()
+
+            # The area reference file might be older, try the photom reference file
+            else:
+                area_ster, area_a2 = self.pixarea_from_ftab(ftab)
 
             # Copy the pixel area values to the output
             log.debug('PIXAR_SR = %s, PIXAR_A2 = %s', str(area_ster), str(area_a2))
@@ -1221,6 +1259,9 @@ def save_area_info(self, ftab, area_fname):
             else:
                 self.input.meta.photometry.pixelarea_arcsecsq = area_a2
                 self.input.meta.photometry.pixelarea_steradians = area_ster
+        else:
+            self.save_area_nirspec(pix_area)
+            pix_area.close()
 
     def save_area_nirspec(self, pix_area):
         """